Variables and Types

Give Your Information a Name

Imagine building a game that remembers a player’s name, score, and whether they’ve completed a level. Rather than typing those values everywhere, you can store them in variables and refer to them by name.

In TypeScript, variables hold information just as they do in JavaScript. The difference is that TypeScript can also check what kinds of values belong in those variables.

const playerName: string = "Alex";
let score: number = 100;

console.log(playerName);
console.log(score);

Here, playerName stores text, while score stores a number. Each variable has a descriptive name that makes the code easier to understand.

When Should a Value Change?

TypeScript uses two common keywords to declare variables: let and const. They look similar, but they have an important difference.

let points: number = 10;
points = 20; // Allowed

const gameTitle: string = "Code Quest";

// gameTitle = "New Title"; // Error

Use let when the variable needs to receive a new value later. Use const when you don’t intend to reassign it.

Notice that changing points doesn’t require writing let again. You declare the variable once, then assign a new value.

Working with Text

A string represents text. You can create strings with single quotes, double quotes, or backticks.

const firstName: string = "Alex";
const language: string = 'TypeScript';

const greeting: string =
  `Hello, ${firstName}!`;

console.log(greeting);
console.log(`Learning ${language} is fun!`);

The backticks create a template literal. Inside one, ${firstName} inserts the value of a variable directly into the text.

Working with Numbers

TypeScript uses the number type for ordinary numeric values, including whole numbers and decimals.

let score: number = 50;
const bonus: number = 25;
const price: number = 19.99;

score = score + bonus;

console.log(score);
console.log(price);

Numbers don’t need quotation marks. If you write "50", you’ve created a string rather than a number.

This distinction matters because adding two numbers is different from combining two strings.

console.log(10 + 5);     // 15
console.log("10" + "5"); // "105"

True or False?

A boolean stores one of two values: true or false. Booleans are useful for tracking conditions such as whether a player is online or whether a task is finished.

let isOnline: boolean = true;
let hasFinished: boolean = false;

console.log(isOnline);

hasFinished = true;
console.log(hasFinished);

Boolean values are written without quotation marks. true is a boolean, while "true" is a string.

Let TypeScript Help You

You’ve seen annotations such as : string and : number. They explicitly describe a variable’s type. But TypeScript can often figure out that type on its own.

let username = "Alex";
let level = 2;
let isReady = true;

username = "Jordan";
level = 3;
isReady = false;

TypeScript infers that username is a string, level is a number, and isReady is a boolean.

let level = 2;

// TypeScript reports an error:
level = "three";

Even without writing : number, TypeScript knows that level should contain a number because its initial value was numeric.

Make Mistakes Before Your Users Do

TypeScript checks assignments against the types it knows. If a value doesn’t match, it reports an error before you run the program.

let attempts: number = 3;

// Error: Type 'string' is not assignable
// to type 'number'.
attempts = "three";

The correct fix is to provide a number:

let attempts: number = 3;
attempts = 4;

console.log(attempts); // 4

TypeScript helps identify mistakes, but it doesn’t replace testing. Type checks happen during development; runtime behavior still needs to be tested.

Build a Developer Profile

Let’s combine everything into a simple developer profile. We’ll store a name, an experience level, and whether the developer is actively learning.

const developer: string = "Alex";
let experience: number = 1;
let isLearning: boolean = true;

experience = experience + 1;

console.log("Developer: " + developer);
console.log("Experience: " + experience);
console.log("Learning: " + isLearning);

That’s the foundation of many real applications: storing information, updating values, and using types to keep everything consistent.